Effect of the surface water content on dry saltation cloud dynamics: A wind tunnel simulation with particle tracking velocimetry

The mass transport rate of wind-borne particles, and indeed the fluid stress required for their entrainment, is strongly governed by inter-particle cohesion arising from water retained through adsorption and capillary force. This paper reports on a series of wind tunnel experiments in which high-speed photography was used to record the motion of dry sand particles as they impinged on test beds of systematically varied target gravimetric pore water content (0% <= W <= 10%). The wind friction velocity was preset to 0.33 m s(-1), sufficient to maintain a saltation cloud above an upwind strip of dry sand, which then was blown over the wetted surface. Discrete particle trajectories were identified in the camera images using expected particle area searching-particle tracking velocimetry (EPAS-PTV). Adding progressively more water produced an exponential decrease in the normalized particle number density over the test surface. The largest response was achieved by wetting the bed surface to just 2%. To reduce the number of particles by a further 40%, it was necessary to add eight times more water, signifying a diminishing return regarding water use. Relative to particles either rebounding or splashed from a dry bed, the total particle velocity increased incrementally by a factor between 1.5 and 2 with increasing water content. Increasing amounts of pore water were associated with progressively higher saltation trajectories, reaching a plateau beyond W similar to 8%. The spatio-temporal adjustment in the sand cloud was observed to be extremely rapid. There is no consensus in the literature on how to measure the water content that effectively governs aeolian transport. In this study, all approaches to sampling W produced strong correlation (R-2 >= 0.85). Sampling the topmost grains, however, provided the most accurate prediction of the normalized number density over the full range of water content.